Although sludge, or a sediment deposited during the treatment of sewage, is sometimes used as a compost, its most frequent use is for land reclamation. Since sludge contains heavy metals and emits an offensive odor and may be decomposed, especially in large cities it is burned and the sludge ashes are used for land reclamation. In addition, recently the sludge ashes came to be used as one of the materials of an earthen pipe or of a tile or as lightweight aggregate.
However, the physical properties of the sludge ashes as used in a ceramic product largely depend upon several factors, including the season when the ceramic product is produced and the method by which the sewage was treated. The sludge ashes thus may more or less determine the qualities of the ceramic product, especially its color, mechanical strength and dimensions. Therefore, for example, to produce ceramic products with substantially the same qualities using ashes obtained by burning sludges which have been obtained by different methods of sewage treatment, suitable materials must be added to the ashes in advance so that the ashes will have substantially the same physical properties when the ashes are in the ceramic products. It takes much time and trouble. Also, a ceramic product containing sludge ashes has not met needs for a wide variety of product colors since Fe.sub.2 O.sub.3 contained in the sludge ashes only gives the product a red brown color or a blackish brown color.